The present invention relates generally to automatic headlight dimming systems for motor vehicles and, more particularly, to a method and apparatus for automatically dimming the headlights of a vehicle wherein a radar is used to detect and identify other vehicles moving within a predetermined area.
Various systems have been developed to provide automatic headlight illumination control in motor vehicles using radar signals. For example, Japanese Patent Document No. 60 64 044 discloses a headlight control device for controlling the illumination range of the headlights based on the speed of the vehicle and the distance to a detected object. The device also changes the optical axis angle of the headlights if an existing illumination pattern and the distance to a detected object indicate that the existing illumination pattern is too bright for an oncoming vehicle at that distance.
During operation, a radar device emits an electromagnetic wave which is reflected by a detected object which can be an approaching vehicle, road sign or other object. The transmission time of the radar signal is used to calculate the distance to the detected object. The illumination distance of the headlights is then regulated based on the distance to the detecting object and the speed of the subject vehicle determined by a vehicle speed sensor.
A significant problem associated with the above described system, and other prior systems, is the persistent occurrence of unwanted changes in headlight illumination caused by stationary objects, such as road signs and guardrails. The above system responds to detected objects solely on the basis of distances of the objects from the subject vehicle. Thus, stationary objects are indistinguishable from approaching vehicles and frequently cause unwanted modulation of the headlight intensity.
Additional problems arise since the above system uses a stationary radar antenna principally focused on the oncoming lane of a two lane highway. Consequently, only objects such as vehicles approaching in a lane adjacent to the driver's side of the vehicle are detected. Vehicles travelling in the same direction or in a nonadjacent lane are not detected and, therefore, do not cause a decrease in headlight intensity. Further, the accuracy of known vehicle speed sensors can vary dependent upon a number of factors including changes in tire size on the vehicle.
It is thus apparent that a need exists for an improved method and apparatus for modulating the intensity of the headlights of a vehicle using radar signals which discriminate between moving vehicles and other objects, measures vehicle speed independently of vehicle factors which influence conventional speed sensors, and detects moving vehicles travelling in the same direction and in nonadjacent lanes.